The soldier, who was deployed with the 1st Marine Expeditionary Force, died on Sunday, the military said in a statement on Monday.
It gave few details on grounds that information could help fighters and put US troops at greater risk.
More than 1150 US troops have been killed in combat in Iraq since the US-led invasion in March 2003 to topple Saddam Hussein.
Armed fighters have been fighting to drive out US-led foreign troops and overthrow the US-backed interim government in Baghdad.
Meanwhile, Iraqi cabinet minister Wail Abd al-Latif said on Sunday he was safe and in his house in Baghdad, after police and Interior Ministry officials said he had been seized south of the capital.
"It's not true. I am safe," Abd al-Latif, minister of state for Iraq's provinces, said.
Police had earlier said Abd al-Latif's convoy had been ambushed south of Baghdad and that he had been seized along with 10 of his bodyguards.
But Abd al-Latif, a Shia judge and ally of interim Prime Minister Iyad Allawi, said all his staff were safe.
There was no immediate comment from police on how Abd al-Latif had been falsely reported captured.
In a separate development, 10 Iraqi civilians, including children and women, were wounded on Monday in a car bomb in al-Dhubbat neighbourhood in central Samara city north of Baghdad, Aljazeera learned.
Most of them were seriously wounded, a medical source said.
Also on Monday, an explosive hit an Iraqi army patrol in al-Sad street in al-Nazzal neighbourhood east of Falluja city, Aljazeera learned.
US and Iraqi army forces were immediately deployed to the explosion site. Casualties among the Iraqi forces were still unknown.